I think the scene in ESB would still work fine, although very differently from what we're used to. It will go from being a total surprise to being a surprise for only the main character - i.e. what you could call the Scottie Ferguson Narrative. From "What will happen?" [in general] the effect becomes, in Hitchcock's words, "What will he do when he finds out about it?" And that's still pretty cool. We didn't stop enjoying rewatching ep.4 after knowing the reveal in ep.5.

If keeping the surprise for new viewers can be done in a convincing way I would love to see it, of course. Magnoliafan claimed to have a way before he left the board. It would also be pretty cool to have Vader, the new apprentice after Anakin dies, kill the younglings (as suggested in this thread), but it would mean insane amounts of rotoscoping.